Patterning of semiconductor wafers is usually carried out using photoresist masks patterned using UV light, electron beams or X-rays. There is recent interest in newer techniques for fabricating micro- or nanostructures. Many of the new techniques do not involve the use of a photoresist and related chemicals, which is advantageous because of improved cleanliness and reduced processing time.
Proximal probe-based lithography is a resistless lithography technique that can generate very high resolution (even down to the single-atom level), but like other such techniques is a serial process. This makes the process generally suitable for small areas, since the series write process is time-consuming.
Soft lithography is a resistless technique that provides for parallel writing. It therefore offers many advantages over photolithography and other microfabrication techniques especially in the patterning of nonplanar substrates and unusual materials. Almost all soft lithography techniques, such as microcontact printing (μCP), replica molding (REM) and micromolding in capillaries (MIMIC) start from a rigid master against which an elastomeric stamp or mold is prepared by cast molding. The master is fabricated using conventional microlithographic techniques such as photolithography, micromachining or e-beam writing. Typically a master is made from a crystalline Si substrate, which sets certain limitations on the dimensions and morphology of the pattern. For instance, molds that contain several different feature heights require multiple applications of lithography and precise mask alignment between steps. In addition, an isotropic KOH etch of crystalline Si limits the angles of etch to be either 90° or 54.74° made by the intersection of the (100) and (111) faces of silicon.
A prior patent related to this work is U.S. Pat. No. 5,318,676. That patent concerned the transfer of projected images into a silicon substrate. The images remained in the substrate and are useful, for example, for optical data storage and emissive circuits requiring a variety of emission wavelengths. The present invention extends the work in the '676 patent to create free-standing three-dimensional patterns of generally arbitrary complexity in silicon.